This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Nitric oxide (NO) is an important signaling molecule, and it binds to the metal center in heme proteins such as myoglobin (Mb), hemoglobin, and cytochrome P450. The inorganic chemistry of the Fe-NO link is fairly well developed, however, the Fe-NO units in heme proteins are sometimes not very well defined in terms of their structural conformation and thus reactivity. We have recently published the 1.9 ang structure of horse heart MbNO (in 2003). During subsequent experimental work, we found that there were differences in the observed Fe-N-O angle in MbNO depending on how the compound is made (whether from nitrite (NO2) or from NO gas). The compound formed from nitrite gives reproducible FeNO angles of 145 deg, and the compound formed from NO gas gives reproducible angles of about 117 deg. To the best of our knowledge, this is the first time that such an observation has been made: that the Fe-NO geometry is dependent on the method of preparation. We thus propose to obtain higher resolution data to better determine the positions of the Fe, N, and O atoms of the FeNO moiety to more accurately determine the differences in the geometries (bond angles, distances, torsion angles, etc). SAMPLES: We will send a set of four crystals for the first method (NO gas method) and a set of six crystals for the alternate method (nitrite method). All crystals have been screened in-house. If possible, we would like data collection on all the crystals. This work will be published in a high-quality journal, and all coordinates deposited at the PDB at the time of manuscript submission.